This invention relates to magnetic bubble (domain) memories and relates in particular to an improved means for enhancing the detection of bubbles or the absence thereof in a detector used in a bubble memory device. More specifically, this invention relates to a means of enhancing the signal generated by such detector so that a signal representing the absence of a bubble in the detector is clearly distinguishable over a bubble presence signal in the detector.
This invention as disclosed in connection with a magnetoresistive bridge type detector in which the absence or presence of a magnetic bubble is determined by a change in resistance in the magnetoresistive material that is producing an output signal accordingly, but this invention will operate equally satisfactory with any type of bubble detector which produces an output signal to be sensed by a differential sense amplifier of the type having a clamp circuit to select the voltage level with which the signal is to be compared at a sampling time and a strobe circuit for sampling the signal at the preselected time.
There are numerous examples of magnetoresistivedetectors in bubble memory chips which rely on a change of resistance of the magnetoresistive material, such as Permalloy, to detect the presence or absence of a bubble propagated thereunder by a rotating in-plane magnetic field and to generate a signal which is sensed by a readout device, usually a differential amplifier, such as the Motorola differential amplifier MC 1544L/1444L. Such detector devices are often arranged in a bridge network in which one magnetoresistive detectors thereof comprises an active detector and another leg thereof comprises a dummy detector; the former being the detector under which bubbles are propagated and the other detector, while subject to the same in-plane rotating propagating field, is out of the bubble path and is for the purpose of providing a noise signal approximately the same in amplitude, frequency and phase as the signal representing the absence of a bubble in the active leg of the detector. The other two legs of the bridge network may be either balancing resistors or active and dummy detectors of other chips so that the current through each leg of the bridge is the same. A typical example of bridge networks using magnetoresistive devices is shown in the U.S. Pat. No. 3,833,858 to Lienhard et al. and in the U.S. Pat. No. 3,953,840 to Cutler et al.
In the foregoing prior art, and especially in the patent to Cutler et al., the signal from the bridge network is sensed by a sensing circuitry which includes a clamp circuit to select the voltage level with which the signal is to be compared, a differential amplifier, and a strobe circuit to sample the signal at preselected times; all of which are means to rid the desired output signal generated by the bubble of noise, whatever the source.
The aforementioned Motorola sense amplifier is used often in bubble memories and is said to have been "originally designed especially for plated wire memories" by Michaelis and Bonyhard in their article entitled, "Magnetic Bubble Mass Memory - - - Module Design and Operation," IEEE Transactions on Magnetics Vol. MAG 9 No. 3, Sept. 1973, Pages 436-441. This sense amplifier includes a clamp circuit (capacitor restore) to set the voltage level at which the signal is to be compared and a strobe circuit to sample the signal at selected time. However, whether the signal processing circuitry of the Cutler et al patent, or the Motorola differential amplifier, supra, is used, the problem is distinguishing a binary "0" signal (ie, the absence of a bubble in the detector, utilizing positive logic) over circuit noise which may be high enough to cause the output signal to be erroneously interpreted as a binary "1" signal. This difficulty is not overcome by increasing the gain of the amplifier since this only serves to increase the noise level also.
Accordingly, it is an object of this invention to improve magnetic bubble memory detection devices by enhancing the output signal therefrom so that the absence or presence of a bubble in the sensor is clearly distinguishable.
A more specific object of this invention is to provide a dynamic offset signal in the signal processing circuitry between the output signal from the detector and the input to the differential sense amplifier so that the sense amplifier can clearly distinguish the signal representing the absence of a bubble from a signal representing the presence of a bubble in the detector.
Still, a more specific object of this invention is to provide a means for enhancing the signal at the input to the differential sense amplifier by lowering the noise level with a negative going pulse so that the sense amplifier, which is unresponsive to a negative signal, can clearly distinguish a binary "0" (bubble absence) from a binary "1" (bubble presence).